Reproduction utilizing a bichargeable photoconductive layer containing zinc oxide and titanium dioxide

ABSTRACT

An electrophotographic recording sheet having a bichargeable photoconductive layer extending over a sheet substrate including as photoconductive pigments in the layer a blend of photoconductive zinc oxide and photoconductive titanium dioxide, the titanium dioxide comprising from 20 to 70 percent of such blend and the zinc oxide comprising at least 20 percent of the blend. These impart to the sheet a positive and a negative charge acceptance exceeding 350 volts, and a positive and a negative light sensitivity value of less than 120 footcandle-seconds.

United States Patent Brandon [4 1 Apr. 4, 1972 [73] Assignee:

Richard L. Brandon, Vancouver, Wash.

Crown Zellerbach Corporation, San Francisco, Calif.

[22] Filed: Mar.11, 1970 [21] Appl.No.: 18,480

LIGHT SENSITIVITY 3,245,785 4/1966 Miller et al ..96/L8 X 3,380,823 4/1968 Gold ..96/1.8 X

FOREIGN PATENTS OR APPLICATIONS Great Britain ..96/1.8 908,779 10/1962 Great Britain Primary Examiner-George F. Lesmes Assistant Examiner-John R. Miller Attorney1(0lisch and Hartwell [5 7] ABSTRACT An electrophotographic recording sheet having a bichargeable photoconductive layer extending over a sheet substrate including as photoconductive pigments in the layer a blend of photoconductive zinc oxide and photoconductive titanium dioxide, the titanium dioxide comprising from 20 to 70 percent of such blend and the zinc oxide comprising at least 20 percent of the blend. These impart to the sheet a positive and a negative charge acceptance exceeding 350 volts, and a positive and a negative light sensitivity value of less than 120 footcandle-seconds.

4 Claims, 2 Drawing Figures POS/T/ VE NEGAT/ V5 [52] U.S.C1. ..96/1 PC, 96/l.8, 96/1.7, 252/501 [51 1 Int. Cl. ..G03g 5/00 [58] Field otSearch ..96/1.8; 252/501 [56] References Cited UNITED STATES PATENTS 3,220,830 11/1965 Kashiwabara ..96/1.8 X 3,245,784 4/1966 Strickland ..96/1.8 X

v; Q g 300 U '-u 9 kl 3 2 200 6 O la 6O 80 Too Patented April 4, 1972 POS/T/ VE CHARGE ACCEPTANCE E W o T o A 7 c E N 0 V! 8 T v w I H 2 w 0 5 w 6 N E E V S F m 0 A T 4 e H E G N L m n C0 o o o o m 0 w w w m w m mzobwm wqQz (U koohx INVENTOR F11 i'ys.

REPRODUCTION UTILIZING A BICHARGEABLE PHOTOCONDUCTIVE LAYER CONTAINING ZINC OXIDE AND TITANIUM DIOXIDE This invention relates to an electrostatic recording product, and more particularly to an electrophotographic recording product which is capable of accepting either a positive or a negative charge over its face, and of making prints with either type of charged condition established, through exposure of selected regions of the product to light in an electrophotographic printing process.

A copy sheet of this description i.e., a dual or bichargeable copy sheet, is useable in producing a positive print (black on white) from either a positive or a negative original (or, if desired, a negative from either type of original). The production of a positive from either a positive or a negative original may be done without changing the developer substance or toner utilized in the printing process. As a consequence, the same copier machine may be utilized in making prints from either type of original, be it negative or positive, without a lengthy shut-down period being required when changing from one type of original to another.

Describing a conventional electrophotographic printing process, utilizing a positive original such as a film strip containing a positive image, a copy sheet first has a uniform negative charge applied over its face, and the sheet is then exposed to light passed through the film strip. Nonimage areas receive light, and light is blocked from image areas. The light sensitizes the photoconductor in the copy sheet rendering it conductive in nonimage areas. This results in dissipation of the charge in areas exposed to light, with an image defining charge being left in areas not receiving light. To develop the print, a positively charged toner is applied to the face of the copy sheet, which adheres to the copy sheet in negatively charged regions.

To produce a print using conventional procedures from a negative original, the copy sheet again has a negative charge uniformly applied over its face. On exposure to light through the negative, charge is dissipated in image areas and remains in nonimage areas. A print can be prepared from such a sheet using a negatively charged toner with such collecting in the image areas where the charge has been dissipated.

The necessity for changing the type of toner utilized has been a serious detriment to the use of the same copier machine for making prints from either negatives or positives. The alternative to changing toner is to provide duplicate copier machines, which, of course, is not economically justified except in operations where a considerable number of both types of prints are produced.

It is recognized that printing processes have been developed wherein initially a recording or copy product is provided with a uniform positive rather than a negative charge. For instance, selenium has been used in a recording product which has been charged in this manner. Such a recording product is relatively expensive, and the general process is not applicable to a disposable product such as a paper copy sheet. Certain organic photoconductors have been proposed which can be charged both positively and negatively, but such have not been generally acceptable in the production of a dual chargeable copy product, by reason of the relatively low sensitivity of the photoconductors, which makes necessary a relatively long exposure to a large amount of light for a charge to be dissipated in a region where such is desired.

This invention is based on the discovery that a highly satisfactory. dual chargeable recording product or sheet may be prepared utilizing a blend of zinc oxide and titanium dioxide as inorganic photoconductive pigments making up a photoconductive layer in the product. It is important that at least about by weight of such blend comprise titanium dioxide to provide proper positive as well as negative charge acceptance. Also important is that the blend contain at least about 20 percent zinc oxide, to provide proper sensitivity in the sheet. In this connection, it has been noted that when a blend of photoconductors of the type described is used, a sensitivity results which exceeds the sensitivity expected as a contribution of each material.

Generally, an object of the invention, therefore, is to provide a novel dual or bichargeable recording or copy product for use in electrophotographic printing processes.

More specifically, an object of the invention is to provide such a recording product prepared from inorganic photoconductive materials.

Another object is to provide a recording sheet capable of being used in conventional copier machines in the production of a positive print from either a positive or a negative original without the need of changing the toner used in making such prints.

Other objects and advantages of the invention will become more fully apparent as the following description is read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a graph illustrating charge acceptance in volts plotted against percent titanium dioxide, in a pigment blend comprising titanium dioxide and zinc oxide that forms the photoconductive coating of a copy sheet as contemplated by the invention; and

FIG. 2 is a graph illustrating sensitivity, in terms of footcandle-seconds (required to obtain a voltage reduction to 40 volts) plotted against percent titanium dioxide, in a blend comprising titanium dioxide and zinc oxide forming the photoconductive coating of a copy sheet.

As already indicated above, this invention contemplates a product which can, be charged either positively or negatively,

prior to having an image prepared thereon using elec-i trophotoconductive techniques. The advantage of such a product is that either a negative or a positive original may be used to produce a print, without changing the toner employed and merely by changing the polarity of the corona which charges the paper prior to preparing an image thereon.

In producing a copy product in the form of a paper copy sheet, a paper substrate is provided which can take the form of any of the usual papers employed in printing products. Thus, the paper may be prepared from a kraft pulp, bleached or unbleached, a ground wood pulp, a sulfide pulp, or other types of chemical or mechanical pulps.

As is recognized in the manufacture of electrostatic recording papers, the substrate selected should have a degree of conductivity to enable charge dissipation in light sensitized areas, and to enable dissipation of stray charges inadvertently acquired by the sheet during its processing or handling, thus to enable the production of the clearest possible prints. The conductance of paper is usually reported in terms of resistance in ohms per square centimeter, or simply ohms per square. With the recording paper of the invention, proper conductivity is obtained in the substrate if such has a resistance which is no higher than about 1 X 10 ohms per square (such being reported at 50 percent relative humidity.

A paper substrate may be prepared with requisite conductivity by including in the pulp from which the paper is prepared a water-soluble conductive salt or other material imparting conductance. Alternatively, such materials may be introduced after production of the paper, by including them in a coating composition applied to the paper, using procedures well known in the art. While in a preferred embodiment of the invention it is believed that paper will be employed as a substrate, it is recognized that other materials can be so employed as a substrate, such as aluminum foil, etc.

Prior to preparing a photoconductive layer with such overlying the substrate, by coating the substrate with a photoconductive coating composition, the substrate preferably is treated for solvent hold out. This is to prevent sorption of the solvent used in the photoconductive coating into the substrate and results in the production of clearer prints. The hold out coating may also be the one including the material imparting conductance. Exemplifying a hold out coating with conductance is one prepared from pigments such as a mixture of clay and titanium dioxide (the latter being a nonphotoconductive or pigment grade included for brighteners) with starch, latex or polyvinyl alcohol as a binder, and containing an inorganic or organic conductive salt. Coating weights are typically one to two pounds per ream and are usually applied to both sides of the paper.

The photoconductive layer is prepared on the substrate by spreading on the substrate a coating composition, applied ordinarily in spreads ranging from about 8 to 24 pounds per ream (3,000 square feet).

The photoconductive coating composition comprises essentially a blend of photoconductive pigments, and a binder, which functions to hold the pigment particles together. Such binders are nonpolar, dielectric materials, having a volume resistivity of 1 X 10 ohms-centimeters or greater. Exemplary of the binders that may be employed are alkyd resins, Saran (copolymer of vinylidene chloride and vinyl chloride), polystyrene, styrene-butadiene copolymers, polyvinyl acetate copolymers, styrene-acrylate copolymers, acrylics, etc. The pigment to binder ratio, i.e., the ratio by weight ofa photoconductive pigment to binder, lies within the range of 2:1 to 9:1, with the preferred range being from 5:1 to 8:1.

As photoconductive pigments in the coating composition, it is contmplated that a blend of photoconductive titanium dioxide and zinc oxide be utilized, with the percent by weight of titanium dioxide in such blend ranging from about percent to about 60 percent, the remainder being zinc oxide. Preferably the percentage of titanium dioxide lies within the range of to 50 percent. By including at least 20 percent by weight titanium dioxide in the blend, proper positive charge acceptance 'is obtained in the coating. The upper limit of titanium dioxide is the blend is dictated by the necessity for a minimal amount of 20 percent zinc oxide to ensure proper sensitivity in the coating.

Further explaining, to obtain clear prints with lines sharply delineated and utilizing conventional toners, it is recognized that a charge of preferably 350 volts or more be capable of being prepared on the coating. The ability of a coating to accept such a charge is referred to as the charge acceptance thereof. Zinc oxide has been recognized as a material capable of accepting a negative charge. By the inclusion of at least 20 percent photoconductive titanium dioxide, in combination with zinc oxide, positive charge acceptance is also obtained, and surprisingly, the charge acceptance is greater than that which would normally be expected as the contribution of the titanium dixoide alone.

The sensitivity of a photoconductor may be defined as the amount of light, or the product of exposure time times light level (footcandle-seconds), needed to reduce the maximum charge that a sheet will accept to a predetermined level. In electrophotoconductive printing, a recognized level to which a surface charge must be dropped to obtain lack of background in a print, is 40 volts and throughout this application this level is the one considered when reference is made to sensitivity. With the usual copier machines, there is a limit to the amount of light available for illuminating a recording sheet and obviously there is a practical limit to the exposure time to which a sheet may be subjected. I have determined that a recording medium should have a sensitivity of no more than about 120 footcandle-foot seconds, to lower the level of surface charge to 40 volts, if the same is to be suitable from a practical standpoint for copier machines.

It is conventional with photoconductors to mix therewith sensitizing dyes, which have the effect of increasing the sensitivity of the photoconductors to visible light. As explained in the literature, this sensitizing of a photoconductor by a dye is accomplished by light first energizing the dye, with the dye then going to a higher energy level. The dye at such higher energy level transfers energy to the photoconductor, with this transfer then being effective to impart conductivity to the photoconductor. Both the titanium dioxide and zinc oxide are sensitized with dyes that absorb light which is in approximately the visible spectrum (4,000 to 7,000A.).

In this connection it has been noted that whereas the sensitivity of a coating comprising photoconductive titanium dioxide alone is relatively poor, where a blend of photoconductors comprising zinc oxide and titanium dioxide is utilized, a sensitivity results which is significantly better than that which would be expected as a contribution of each of the photoconductors alone. While the reason for this is not fully understood, it is felt that such can at least partially be explained by the fact that in such blends, zinc oxide functions to absorb any sensitizing dye, with such on absorbtion being so closely associated with titanium dioxide as to enable an energy transfer to take place with the titanium dioxide. Further explaining, it is relatively difficult to obtain dye absorption by titanium dioxide, however, with a blend of titanium dioxide and zinc oxide, and with absorption of the dye by zinc oxide as above set forth, an effect similar to the one which would result were the titanium dioxide to absorb dye is produced.

The photoconductive zinc oxide and titanium dioxide employed in the instant invention are each pigments that produce, when incorporated alone into a photoconductive film or layer spread over a substrate, a charge acceptance in such layer of at least about 350 volts. The charge acceptance in the case of zinc oxide is a negative charge acceptance, and in the case of titanium dioxide a positive charge acceptance. In making such a determination of charge acceptance, :1 styrene-ethylacrylate copolymer is the binder employed in preparing the photoconductive layer, a pigment to binder ratio of 7:1 is used and a sensitizing dye is present comprising 70 p.p.m. (based on weight of pigment) of fluorescein and 48 p.p.m. bromophenol blue. A coating weight of 20 pounds per ream is further utilized. The photoconductive zinc oxide demonstrates in such a photoconductive film or layer a sensitivity of at least about 150 footcandle-seconds (such being a negative charge sensitivity), and the photoconductive titanium dioxide demonstrates a positive charge sensitivity of at least about 500 footcandle-seconds. Photox 80, which is a French process zinc oxide available commercially from the New Jersey Zinc Company, exemplifies a photoconductive zinc oxide meeting this specification (sensitivity approximately 3 to 4 footcandle-seconds, charge acceptance 495 volts). A photoconductive titanium dioxide pigment available from Titanium Pigment Corporation, and prepared as set forth in Netherlands publication Octrooiaanvrage US. Pat. No. 6,812,232, filed Mar. 4, 1969, exemplifies a photoconductive titanium dioxide pigment meeting such a specification (sen sitivity about 365 footcandle-seconds, charge acceptance 6 l0 volts).

Exemplary of the dyes that have been utilized in sensitizing the photoconductive blends herein contemplated, are Eocin J, Rhodamime B, bromophenol blue, Bromocresol purple, Acridine orange, Alizarine Red S, Pinacyanol chloride, Patent blue V, crystal violet, Chrom Black T, and fluorescein.

The following examples illustrate the invention as specifically practiced.

EXAMPLE 1 A photoconductive coating composition was prepared from the following: 17 parts binder (styrene-ethylacrylate copolymer); 84 parts photoconductive zinc oxide (Photox-80,

New Jersey Zinc Co.); 36 parts photoconductive titanium dioxide (of the type earlier described obtained from Titanium Pigment Corporation); parts solvent (toluene).

The above ingredients were mixed by ball milling, for a period of 5 hours, to obtain complete and thorough dispersion of the photoconductive materials or pigments. Sensitizing dye was then added to the coating composition, the dye comprising 1 percent fluorescein in methanol and 1 percent bromophenol blue in methanol, in sufficient quantity to introduce 7O p.p.m. fluorescein and 48 p.p.m. bromophenol blue (based on pigment weight). Such was mixed for an additional half hour by the ball milling.

Paper (40 pound/ream kraft paper) had hold out coatings applied to opposite faces using spreads of 2 pounds per ream. The hold out coating composition was an aquious mixture containing, for parts of water, 90 parts clay, 10 parts pigment grade titanium dioxide, 15 parts starch, and 30 parts conductive polymer (poly N,N-dimethyl-3, 5 methylene piperidinium chloride sold as Calgon 261). The paper substrate which resulted had a conductivity of l X 10 ohms per square.

The photoconductive coating composition was applied as a coating over one side of the paper substrate. A recording or copy sheet cut from such coated paper was charged positively, with a positive corona discharge. The sheet accepted a positive charge of 445 volts. The same sheet was charged negatively with a corona discharge of negative polarity, and accepted a charge of 505 volts.

The recording sheet, with such positive charge, was exposed to a tungsten light source. 6.2 footcandle-seconds were required to reduce the surface charge of the sheet to 40 volts. The same sheet, with a negative charge over the face thereof, was exposed to a tungsten light source and 10.7 footcandleseconds were required to reduce the surface charge of the sheet to 40 volts.

A copy sheet prepared as set forth above was imaged in an electrophotographic copier machine. The master utilized was a negative master film (white on black). Imaging was done by first positively charging the sheet, using a positive corona discharge. The imaged sheet was developed with a positively charged toner, to produce a good quality, positive print.

Another sheet prepared as set forth above was imaged in an electrophotographic copier machine, using a positive master film (black on white). Imaging was done by first negatively charging the sheet using a negative corona discharge. The imaged sheet was developed with a positively charged toner, to produce a good quality, positive print.

A series of copy or recording sheets were prepared as set forth in Example I, using different proportions of photoconductive titanium dioxide and zinc oxide in the blend making up the photoconductive coating composition. Sheets so I prepared were charged, both positively and negatively, to

determine their charge acceptance, and exposed to a tungsten light source, to determine their sensitivity in terms of the light required to reduce their surface voltage to 40 volts. The results obtained on analyzing such sheets are summarized in the following table.

FIG. 1 is a graph in which charge acceptance, both positive and negative, is plotted against ratio of titanium dioxide in the blend of photoconductive pigments contained in the photoconductive coating. It will be noted from a review of such figure that a charge acceptance, both positive and negative, exceeding 350 volts, is obtainable with blends comprising percent or more titanium dioxide. Further, and referring to FIG. 2 which plots sensitivity against percentage of titanium dioxide in the photoconductive coating composition, it will be noted that with blends of less than about 80 percent titanium dioxide a sensitivity of I20 footcandle-seconds or lower is obtainable.

In preparing the recording sheets, it has been noted that best results are obtained if the zinc oxide and titanium dioxide are well dispersed with the binder and other ingredients. Thus, it has been noted that preferably either a ball milling or a sand milling is performed in making the composition. The milling which is performed is one which preferably is at least the equivalent with respect to a blending action of 4 hours of ball milling. With such a mixing procedure, good image density is obtained, together with good resolution. If milling is continued for too great a time, i.e., for a period of greater than about hours of ball milling, resolution is good but image density suffers.

As perhaps best illustrated by reference to FIG. 2, it will be noted that with the photoconductive layer containing a blend of photoconductive materials, as contemplated by the inventron, a sensitivity results which IS greatert an the sensitivity to be expected as contributed by each of the components in the blend. For example, with a blend containing 50 percent titanium dioxide and 50 percent zinc oxide, the expected sensitivity would be one about midway between the sensitivity of the two ingredients when used alone, i.e., a sensitivity of approximately footcandle-seconds. Quite surprisingly, however. the sensitivity of the blend more nearly approaches the sensitivity of zinc oxide, both with respect to a negative charge and to the positive charge. This sensitivity moreover, is with a positive charge acceptance, as demonstrated in FIG. 1, which is almost the same as the negative charge acceptance of the copy sheet, i.e., a charge acceptance in the neighborhood of 500 volts in each instance.

It is claimed and desired to secure by Letter Patent: 1. A method of electrophotographic printing which produces the same type of printed copy sheets on duplicating alternatively positive and negative originals, the method comprising applying a uniform charge to recording sheets faced with a bichargable photoconductive layer which includes as photoconductive pigments a blend of photoconductive zinc oxide and photoconductive titanium dioxide, the titanium dioxide comprising from 20 to 70 percent of such blend and the zinc oxide comprising at least 20 per cent of the blend, said pigments imparting to said recording sheets a positive and negative charge acceptance ex ceeding about 350 volts and a positive and negative light sensitivity value of less than about I20 foot candle seconds, said charge being applied to the faces of the recording sheets and said sheets having a substrate with a resistance no greater than about 1 X 10 ohms per square,

the charge applied having one polarity in the case of duplicating a positive original and the opposite polarity in the case of duplicating a negative original,

exposing the recording sheets to light using the negative and positive originals to control light delivery to the copy sheets and to produce charge dissipation in image areas of a recording sheetwith duplication of one type of original and charge dissipation in nonimage areas of a recording sheet and retention in image areas with duplication using the other type of original, and

developing the recording sheets as so prepared to produce printed copy sheets by distributing charged toner of the same polarity over the sheets, with such toner collecting in charged areas of a recording sheet where such sheet has charged areas having a polarity which is opposite to the polarity of the toner, the collecting in areas of no charge in a recording sheet where such sheet has charged areas having a polarity which is the same as the polarity of the toner.

2. The method of claim 1, wherein developing of the recording sheets is done by distributing a toner having a polarity opposite to the polarity of image areas in a sheet where charge is retained in image areas after light exposure, such toner having the same polarity as the polarity of the charge in nonimage areas on a recording sheet where charge is dissipated in image areas.

3. The method of claim 2, wherein the charge initially applied to the faces of the recording sheets has a positive charge in the case of the duplication of a negative original, and a negative charge in the case of the duplication of a positive original, and a positively charged toner is used in developing the recording sheets.

4. The method of claim 1, wherein the photoconductive layer includes a photoconductive sensitizing dye for each of the pigments in the blend, which dye is intimately dispersed with the pigments in the blend, and exposure of the recording sheets to light is accompanied with sensitizing of the photoconductors by such dye. 

2. The method of claim 1, wherein developing of the recording sheets is done by distributing a toner having a polarity opposite to the polarity of image areas in a sheet where charge is retained in image areas after light exposure, such toner having the same polarity as the polarity of the charge in nonimage areas on a recording sheet where charge is dissipated in image areas.
 3. The method of claim 2, wherein the charge initially applied to the faces of the recording sheets has a positive charge in the case of the duplication of a negative original, and a negative charge in the case of the duplication of a positive original, and a positively charged toner is used in developing the recording sheets.
 4. The method of claim 1, wherein the photoconductive layer includes a photoconductive sensitizing dye for each of the pigments in the blend, which dye is intimately dispersed with the pigments in the blend, and exposure of the recording sheets to light is accompanied with sensitizing of the photoconductors by such dye. 